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    <title>Document</title>
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    <script>
        // 基本数据类型
        // number string boolean undefined null
        var a;//undefined
        var b=20;//number
        var c=true;//boolean
        var d=false;//boolean
        // 用单引号和双引号包裹的就叫string
        var str="hello"//string
        var str_one='good';//string

        // typeof 识别一个数据类型
        console.log(typeof a);
        console.log(typeof b);
        console.log(typeof c);
        console.log(typeof d);
        console.log(typeof str);
        console.log(typeof str_one);
    </script>
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